Emissions of Black Carbon Aerosols from Alaskan Boreal Forest Wildfires

Friday, 19 December 2014
Gergana Mouteva1, Simon M Fahrni2, Brendan M Rodgers3, Elizabeth Brooke Wiggins4, Guaciara Santos5, Claudia I Czimczik1 and James Tremper Randerson6, (1)University of California Irvine, Irvine, CA, United States, (2)ETH Swiss Federal Institute of Technology Zurich, Zurich, Switzerland, (3)Woods Hole Research Center, Falmouth, MA, United States, (4)University California Irvine, Tifton, GA, United States, (5)Univ of California Irvine, Irvine, CA, United States, (6)University of California Irvine, Department of Earth System Science, Irvine, CA, United States
Boreal wildfires are a major source of carbonaceous aerosols. Emissions from wildfires in Alaska represent ~ 33% of all open biomass combustion emissions of black carbon (BC) in the United States. BC contributes to atmospheric warming and accelerates melting of ice and snow. With fire frequency and burned area projected to increase in boreal regions, BC has the potential to become an important positive feedback to climate change. Quantifying the emissions, constraining the sources and better understanding the transportation patterns of BC to the polar regions are therefore critical for constraining the strength of this feedback. We present results from direct measurements of BC from wildfires in Alaska during the summer of 2013 collected as a part of NASA’s Carbon in Arctic Reservoirs Vulnerability Experiment (CARVE) campaign. Fine aerosol particulate matter (PM2.5) was collected at two locations: Caribou-Poker Creek Research Watershed and Delta Junction Agricultural and Forestry Experimental Site. Using a Sunset OCEC analyzer, we separated BC from organic carbon aerosols, measured concentrations and analyzed the radiocarbon (14C) content with accelerator mass spectrometry. We also analyzed the total carbon (C) and nitrogen (N) elemental and stable isotope composition of the bulk PM2.5 with EA-IRMS. We compared the temporal dynamics of BC concentrations and isotopic composition with active fire/thermal anomaly information from MODIS. Our results show that boreal forest fire emissions in interior Alaska increased BC concentrations by up to an order of magnitude above background levels. The mean Δ14C value of fire-emitted BC was 120‰ with a range of +99‰ to +149‰ after correcting for contributions from background BC. This range was in good agreement with measurements of the depth of burn in soil organic carbon layers from interior wildland fires, and Δ14C profiles. High fire periods also corresponded to elevated C:N ratios. The δ15N of the aerosols was significantly elevated during high fire periods, and was likely influenced by the depth of burning in the organic soil horizons and isotope fractionation during combustion. Direct measurements of fire-emitted BC and PM2.5 composition provide independent insight into the potential effects of a changing boreal forest fire regime on the climate system.